The invention concerns generally the technology of using small-sized planar multiband antennas in portable telecommunication devices. Especially the invention concerns the technology of tuning such multiband antennas so that their operational frequency bands can be controlled to match certain target values.
An individual operational frequency band of an antenna arrangement, or just an antenna for short, is the continuous range of frequencies through which both the resonance characteristics of the resonant antenna element and the impedance matching between the resonant antenna element and the antenna port of the device coupled thereto are good enough to enable efficient transmission and/or reception. The criteria for both the resonance characteristics and the impedance matching are set in terms of attenuation in decibels, and these are typically laid down in system specifications. A multiband antenna means an antenna that has several operational frequency bands. An electrically tunable antenna is an antenna at least one operational frequency band of which can be repeatedly and controllably shifted along the frequency axis by applying an electric control signal to a certain part of the antenna.
A Planar Inverted-F Antenna, known also by its acronym PIFA, consists of an essentially planar radiating element, a ground plane that is essentially parallel to the radiating element, a feeding pin for coupling the radiating element to a duplexer or some other part of a radio device, and a grounding connection for coupling a certain point of the radiating element to the ground plane. The grounding connection is also called a short-circuit to ground or just a short. It is known that multiband PIFAs can be produced by making branches of different dimensions to the radiating element and by placing the feeding pin at a suitable location with respect to the branches. Another known way of producing multiband planar antennas is to use a number of independently fed planar radiating elements above a common ground plane. Prior art publications that disclose multiband planar antennas are known from e.g. EP1026774, EP1024552, U.S. Pat. Nos. 6,072,434, 5,926,139, EP0856907, U.S. Pat. No. 5,764,190 and WO9627219.
The public is continuously demanding a so-called world phone, meaning a single mobile telephone that could be used in all major mobile telephone networks around the world. A considerable hindrance to the task of developing such a phone is the fact that different mobile telephone systems use a variety of frequency ranges. On the other hand there is a clear trend towards smaller and smaller mobile telephones preferably with no protruding antennas. This means that in addition to having several operational frequency bands, the world phone antenna should be exceptionally compact in size in order to fit inside the covers of a modem mobile telephone. With the antenna technology of the priority date of this patent application, these requirements mean that a multiband planar antenna is about the only feasible solution.
Electrically tunable antennas are known from publications EP1014487, U.S. Pat. Nos. 5,943,016, 5,777,581, EP0993070, JP10224142 and JP9307344. The possibility of electrically tuning the antenna represents a remarkable advance towards a compact multiband antenna, but even these solutions leave open certain questions regarding how should the tuning control be implemented. Additionally there is the problem of how should the mobile telecommunications device react when it enters an area where a serving cellular radio system employs a different operational frequency band than the one onto which the antenna of the mobile device has currently been tuned.
It is an objective of the present invention to provide an antenna arrangement that has multiple operational frequency bands and tuning means for tuning these operational frequency bands to required locations on the frequency axis. It is a further objective of the invention that the antenna arrangement is compact in size and suitable for mass production. Another objective of the invention is to make it easy to adapt the tuning to situations where tuning should be changed for some reason. A yet another objective of the invention is to solve the problem that arises when the tunable antenna has been tuned to a different operational frequency band than that which a cellular radio system is currently using.
The objectives of the invention are achieved by providing a planar radiating element and multiple controllable grounding connections from the planar radiating element to an adjacent ground plane, and by dimensioning the electric characteristics of the grounding connections in a suitable way. The objectives are further achieved by arranging the control of the grounding connections in an advantageous way. A yet other aspect of achieving the objectives of the invention is to use a fixed wideband antenna for receiving.
The antenna arrangement according to the invention is characterised by the features recited in the independent patent claim directed to an antenna arrangement. Additionally the invention applies to a mobile telecommunications device the characteristic features of which are recited in the independent patent claim directed to a mobile telecommunications device.
Electrically tuning an antenna is not novel as such. It has been implemented e.g. by placing various conductive tuning elements in the vicinity of a radiating element and using electrically controllable semiconductor switches to open and close the connections between the tuning elements and the radiating element in a controlled manner.
According to the invention, it is not feasible to try to cover all required frequency ranges of a mobile telephone operable for reception and transmission in a multitude of mobile telephone systems by building an antenna so that it would inherently have operational frequency bands that would match each required frequency range. A multiband planar antenna can be constructed by using techniques known as such by making the planar radiating element to consist of at least two branches. A relatively simple but highly effective and reliable tuning arrangement then consists of a multitude of differently dimensioned and controllably switched grounding connections between the planar radiating element and an adjacent ground plane that can be opened and closed in various combinations.
In a preferred embodiment of the invention there are three controllably switched grounding connections relatively close to each other between the radiating element and the ground plane. The term xe2x80x9ccontrollably switchedxe2x80x9d is interpreted widely so that it covers even a case where one of the connections is permanently in an xe2x80x9cONxe2x80x9d state, i.e. does not actually need to have a switch at all. Each grounding connection represents a certain impedance between the (approximately common) coupling point at the radiating element and the ground potential. The impedance consists of the series resistance of the switching component, a tuning capacitance, a parasitic inductance of the switch package and the inductance of the grounding connection itself. Various combinations of carefully dimensioned impedances result in an antenna arrangement where certain states of the switches on the grounding connections correspond to matches between the operational frequency bands of the antenna arrangement and the frequency ranges of the required mobile telephone systems.
The problem of inherently narrow impedance bandwidth of multiband planar antennas is most advantageously circumvented so that the antenna according to the invention is only used in a mobile telephone (or more generally: in a mobile telecommunications apparatus) for transmitting. A separate antenna can be used for receiving. Such an arrangement loosens considerably the requirements for providing a high-quality versatile duplexer. Using separate antennas for transmission and reception also allows optimization to be made separately for both modes. The invention does not exclude using the antenna according to the invention for both transmission and reception, or even using two antennas according to the invention so that one is used for transmission and the other for reception.
An advantageous feature of the electrically tunable antenna arrangement according to the invention is that by using an appropriate feedback loop, the tuning can also be utilized for compensating for imperfections in antenna operation that are caused by the user""s hand being placed too close to the radiating antenna element.